In the past, there is known a contact apparatus that includes, as shown in FIG. 18, fixed terminals 33 with fixed contacts 32, a movable contact member 71 with movable contacts 72, a yoke plate 81, a contact pressure spring 36, a movable shaft 91, a case 31, and a drive means 2 (see, e.g., Japanese Patent Application Publication No. 2010-010056).
The movable contact member 71 is formed into a substantially rectangular plate shape. The movable contacts 72 are arranged in the left and right end regions of the upper surface of the movable contact member 71. An insertion hole 71a is formed in the substantially central region of the movable contact member 71.
The yoke plate 81 is made of a magnetic material and formed into a rectangular plate shape. The yoke plate 81 is fixed to the movable contact member 71 with the upper surface of the yoke plate 81 kept in contact with the lower surface of the movable contact member 71. The yoke plate 81 has an insertion hole 81a formed in the substantially central area thereof.
The movable shaft 91 includes a rod-shaped shaft portion 911 movably inserted through the insertion hole 71a of the movable contact member 71 and the insertion hole 81a of the yoke plate 81 and a yoke contact portion 912 having a rectangular plate shape which is made of a magnetic material and fixed to the upper end of the shaft portion 911.
The yoke contact portion 912 is formed to have a thickness substantially equal to the thickness of the yoke plate 81. The yoke contact portion 912 faces the upper surface of the movable contact member 71 and restrains the movable contact member 71 from moving toward the fixed contacts 32. The yoke contact portion 912 faces the yoke plate 81 through the movable contact member 71.
The contact pressure spring 36 is formed of a coil spring. The shaft portion 911 of the movable shaft 91 extends through the bore of the contact pressure spring 36. The upper end of the contact pressure spring 36 makes contact with the lower surface of the yoke plate 81 and presses the movable contact member 71 toward the fixed contacts 32 through the yoke plate 81.
An electromagnet is used as the drive means 2. The lower end of the shaft portion 911 of the movable shaft 91 is connected to the electromagnet.
If the movable shaft 91 is moved upward by the drive means 2 and if the yoke contact portion 912 is moved toward the fixed contacts 32, the movable contact member 71 is released from the restraint of movement toward the fixed contacts 32. Then, the movable contact member 71 is moved toward the fixed contacts 32 by the biasing force of the contact pressure spring 36. The movable contacts 72 come into contact with the fixed contacts 32.
In the contact apparatus, if a large current such as a short-circuit current flows between the fixed contacts 32 and the movable contacts 72, a downward contact repulsion force acts against the movable contact member 71 under the influence of the magnetic fields generated around the movable contact member 71.
In the contact apparatus, however, if the movable contacts 72 make contact with the fixed contacts 32 and if an electric current flows through the movable contact member 71, a magnetic flux passing through the yoke contact portion 912 and the yoke plate 81 about the movable contact member 71 is formed around the movable contact member 71. As a consequence, a magnetic attraction force acts between the yoke contact portion 912 and the yoke plate 81. The contact repulsion force is suppressed by the magnetic attraction force, thereby preventing a reduction in the contact pressure between the fixed contacts 32 and the movable contacts 72.
In the prior art, however, a demand has existed for a contact apparatus capable of obtaining a large contact pressure while suppressing an increase in size.